Substances undergo changes in two fundamental ways: physically and chemically. A physical change alters a substance’s appearance, such as melting ice into water or crushing a rock into powder, but it does not change its core identity. In contrast, a chemical change alters the substance’s molecular structure, creating a new substance with entirely different properties.
Defining Chemical Reactions
A chemical reaction is the process that defines a chemical means of breakdown. This process involves the rearrangement of atoms as existing chemical bonds are broken and new bonds are formed. The starting materials, known as reactants, are transformed into new substances called products, which possess different chemical identities and characteristics than the original reactants.
Breaking the strong forces of attraction that hold atoms together in a molecule requires an input of energy, often referred to as activation energy. This energy can be supplied through various forms, such as heat, light, or electricity, to overcome the stability of the original substance.
Compounds: The Primary Candidates for Chemical Breakdown
Compounds are the substances that must be broken down by chemical means. A compound consists of two or more different elements chemically bonded together in a fixed ratio, such as water (H₂O) or table salt (NaCl). The chemical bonds, which can be ionic or covalent, are strong and require significant energy to break.
Decomposition reactions, a type of chemical reaction, are necessary to break these bonds and separate the compound into its constituent elements or simpler compounds. For example, water molecules are stable and cannot be broken apart by simply boiling them, even at extreme temperatures. To break water down into its elements, hydrogen and oxygen gases, the process of electrolysis is required, which uses electrical energy to overcome the strong covalent bonds.
Another common method is thermal decomposition, where intense heat is applied to a compound, forcing the chemical bonds to break. Calcium carbonate, for instance, decomposes into calcium oxide and carbon dioxide when heated strongly.
Elements: Substances That Resist Chemical Breakdown
Elements cannot be broken down into simpler substances by chemical means. An element is a pure substance consisting of only one type of atom, such as gold (Au), oxygen (O), or carbon (C). Since there are no chemical bonds between different elements to break, a chemical reaction cannot simplify the substance further.
The definition of a chemical reaction involves rearranging atoms, but it does not change the identity of the atoms themselves. Trying to break down an element chemically only results in the same type of atom. Changing one element into another would require altering the number of protons in the atomic nucleus, a process that falls outside the scope of chemistry and into the field of nuclear physics. Therefore, elements are the fundamental building blocks that resist all chemical methods.
Mixtures: Substances Separated by Physical Means
Mixtures are different from compounds because they involve substances that are physically combined but not chemically bonded. In a mixture, each component retains its individual chemical identity and properties. Because no chemical bonds were formed, no chemical reaction is necessary to separate the components.
Instead of chemical breakdown, mixtures are separated using various physical methods that exploit differences in the components’ physical properties. For example, a mixture of saltwater can be separated by evaporation, where the water is boiled away, leaving the solid salt behind. The salt and the water remain chemically the same throughout this process.
Other separation techniques include:
- Filtration, used to separate an insoluble solid like sand from water.
- Magnetic separation, used to pull iron filings out of a mixture with sand.
- Distillation, which separates liquids with different boiling points, such as alcohol and water.